The present invention relates to automatic blood pressure measuring devices. Specifically, an apparatus for automatically measuring and displaying values of systolic and diastolic blood pressure is described.
It is known to make measurements of blood pressure based upon the Korotkoff sound method. In the Korotkoff sound method, the artery of a patient in the upper arm area is pressurized by means of a pressurizable cuff wrapped around the upper arm. The pressurization rate applied to the upper arm area is in the vicinity of 20 mm hg/second and 30 mm hg/second. The pressure applied to the arm area is increased until it is 10 mm hg to 20 mm hg higher than the expected systolic or maximum blood pressure, at which point the arterial flow of blood is completely cut off. In this process, the cuff pressure is decreased gradually at a rate of 2 to 8 mm hg per second until the artery is able to pass a small amount of blood. During the systolic period of the heart beat, the arterial blood flow reaches a maximum speed and passes through the slightly opened artery. As blood flows towards the arterial wall, there is an attenuation vibration in the high frequency pulse wave produced by the blood which constitutes the aforesaid Korotkoff sound. At this time, cuff pressure approximates the systolic blood pressure. Further reduction of the pressure to the arm area produces pulse waves that are in synchronism with the Korotkoff sounds, and at the point where the artery is open and the heart is in the diastolic period, the Korotkoff sounds cease. The cuff pressure at this time expresses the diastolic blood pressure permitting a reading of both the systolic and diastolic blood pressure.
Prior art devices have depended upon detecting the Korotkoff sound pulse with a microphone. When the Korotkoff sound is above a specific output level, the cuff pressure is read to determine the systolic blood pressure. Corresponding to this measurement, the level of the Korotkoff sound signal is monitored to determine when it is below a predetermined level, at which time the cuff pressure is recorded at the diastolic blood pressure. The prior art improved upon detection of the Korotkoff sounds utilizing a first low-range filter, having a band pass between 30 and 50 hertz to extract the systolic Korotkoff sound signal low frequency components. Further, a second filter was employed which extracted the higher frequency components having a frequency between 80 to 120 hertz of the Korotkoff sound to measure the diastolic blood pressure. When the output from the low frequency filter ended, the pressure exerted by the cuff was recorded at the systolic blood pressure. When the output from the second filter ceased, the pressure applied by the cuff was recorded as the diastolic blood pressure.
This method provided for measurement errors due to the differences between the Korotkoff sounds produced by different patients. It occurs between patients that the relative appearance and time of disappearance of the low-range frequency components of the Korotkoff sound signal and the high-range frequency components differ from individual to individual. Referring to FIG. 5, the problems in using this technique are graphically illustrated. FIG. 5a shows the general pattern of the Korotkoff sound signal with respect to the low frequency components of the second signal. The high frequency components, as shown in FIG. 5b, occur somewhat later than the low frequency components and disappeared slightly before the low frequency components. Other individuals, however, have a pattern shown in FIG. 5c wherein the high frequency components disappear later than the low frequency components, and, in FIG. 5d, it is shown that there is still another signal pattern where the high frequency components appear before the low frequency components and therefore are not taken as an indication of the systolic blood pressure.
A further problem in using the prior art technique of blood pressure measurements is illustrated in FIG. 5e where the high frequency components appear slightly later than the low frequency components of the Korotkoff signal. Prior art devices are arranged to begin taking diastolic blood pressure measurements after the occurrence of 4 pulses from the first low frequency filter. The built in time delay presents problems with a pulse pattern of FIG. 5e. A pulse pattern of FIG. 5e would result in the diastolic blood pressure measuring technique having a starting point slightly later than the appearance of the high frequency components, avoiding measurement of the diastolic blood pressure altogether.